The present invention relates to supported catalysts and in particular to supported catalysts for use in the gas phase (co-) polymerisation of olefins in a fluidised bed reactor.
Processes for the co-polymerisation of olefins in the gas phase are well known in the art. Such processes can be conducted for example by introducing the gaseous monomer and comonomer into a stirred and/or gas fluidised bed comprising polyolefin and a catalyst for the polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst becomes inactive or the bed commences to fuse. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent gas such as nitrogen or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas serves to supply the monomer to the bed, to fluidised the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
It is also well known that fouling in gas phase polymerisation process can be a major problem, and can be caused by non-uniform fluidisation as well as poor heat transfer in the polymerisation process. Catalyst and polymer particles may adhere together or to the walls of the reactor and continue to polymerised, and often fuse together and form chunks, which can be detrimental to a continuous process, particularly a fluidised bed process
The incorporation of antistatic agents in polymerisation catalysts is well known. For example U.S. Pat. No. 5,414,064 describes the use of Stadis with chromium based catalysts while U.S. Pat. No. 5,498,581 describes again the use of Stadis with silica supported metallocene catalyst systems.
U.S. Pat. No. 6,469,111 describes the gas phase polymerisation of olefins using a catalyst system containing an antistatic agent based on magnesium oxide or zinc oxide. The oxides are used in a mixture with a supported polymerisation catalyst component. The catalyst systems described therein include supported chromium oxide catalysts and also supported metallocene complexes in particular bis(cyclopentadienyl) metallocene complexes.
More recently WO 02/066524 describes supported catalysts for olefin polymerisation comprising a combination of a sulfated metal oxide support and an aluminoxane. The sulphated metal oxide support may be easily prepared by contacting a precursor metal oxide with a material having a SO4 group such as sulphuric acid or ammonium sulphate. Typically the resultant aluminoxane deposited on the sulphated metal oxide is used with an organometallic complex of a Group 4 metal, in particular with metallocene complexes comprising both a cyclopentadienyl ligand and a phosphinimine ligand.
U.S. Pat. No. 6,107,230 describes catalyst compositions comprising inorganic metal oxides impregnated with metal salts eg. cupric sulfate, metallocenes and organoaluminium compounds. The compositions may be combined together or preferably added separately into the reactor. An important aspect of this disclosure is that traditional activators such as aluminoxanes, borates or magnesium chloride are not required.